1. Field of the Invention
The invention relates to a milling device with double milling passage, to an apparatus which uses the device, and to a method which uses the device.
More particularly, the invention relates to the milling of grain, particularly cereals and derivatives thereof, by feeding the product to be milled to two pairs of milling rollers. The amount of product fed to the rollers is determined, for example, by means of a capacitive sensor, according to Italian patent application MI 98 A 000117. The milling rollers of each pair rotate at mutually different speeds. In this manner, the different mutual speed of the two rollers which occurs in the contact region produces friction which does not simply crush the cereals but pulverizes them. In this step separation between the particles of the outer skin of the grain and the semolina also occurs. The dimensions of the particles obtained by virtue of a milling passage depend on the distance between the rollers, on the moisture, et cetera.
In order to obtain high-quality flour, as required by the market, it is necessary to adequately separate the semolina particles from the outer skin particles. The most effective method consists in sizing semolina batches and then sending them to specific machines which separate by density the pure semolina from the semolina that has not yet been cleaned and from the outer skin parts. These machines are known as plansifters. Optimum efficiency and capacity of these machines are closely linked to the particle size of the semolina to be cleaned; specifically, the larger the particle size of the semolina, the easier it is to separate the outer skin parts from the semolina and the greater the capacity of the machine.
This is the technological reason why flour producers prefer to separate by sifting, immediately after each milling passage, the semolina that has been produced, to prevent the semolina from being subjected to a particle size reduction due to another milling passage.
The plansifter, that is to say, the machine that separates the semolina particles from the outer skin particles, in fact operates according to density. Relatively small semolina particles tend to have, in the powder that forms inside the plansifter, a density which is relatively more similar to the density of the outer skin parts and therefore separation thereof is more difficult. In practice, the ascending air stream which is present in the plansifter and should stratify the particles according to density increases in effectiveness as semolina particle size increases.
In order to solve this problem, a separation stage is generally provided after each milling stage in a cereal milling apparatus. In this manner it is possible to separate the outer skin particles from the semolina particles in a timely fashion, when the semolina particles are still relatively large, before the subsequent milling.
The above is the main field of industrial application of the invention and does not constitute a limitation, since the invention can be used in equivalent fields.
2. Description of the Prior Art
It is known to use cereal roller mills which use two stacked milling passages. Each passage uses two pairs of milling rollers, so that the product is milled first by the two pairs of upper rollers and then by the two pairs of lower rollers. This system is shown for example in European patent EP 334919 in the name of Buehler. In this manner, however, the semolina particles are inevitably milled twice, reducing their dimensions to a level which, for many applications, is considered too small to allow adequate separation.
On the other hand, this solution with two stacked milling passages is useful to increase the productivity of a milling apparatus even in confined spaces, reducing costs.
It has also been noted that whenever a cereal or derivative thereof is passed through a pair of milling rollers, its volume increases, that is to say, its relative density decreases. In conventional milling with sifting after each milling passage, each successive pair of milling rollers is loaded with a smaller amount than the previous milling passage; the increase in volume is therefore compensated by a reduction in the amount of product. In the case of "stacked" milling without intermediate sifting, this balancing is not possible and it is therefore necessary to reduce the capacity of the machine. Attempts have been made to obviate the problem by increasing the rotation rate of the lower passage, but this refinement has limitations, since there are speeds which it is not convenient to exceed, otherwise milling quality worsens. In practice, therefore, a mill with double stacked passages is unable to mill the same amount of flour as a single-passage mill. If the length of the milling generatrix is equal and is approximately 1000 mm, a single-passage mill processes, if used as first break stage, an average of 8 tons per hour of soft wheat, whilst a stacked double-passage mill mills up to 6 tons per hour.
In order to overcome these problems, it has long been thought to insert in a stacked double-passage mill, an intermediate sifting system arranged between the first passage and the second passage. This solution is shown in patent GB-A-6693 dated 1908 in the name of Simon and in the French patent 415.230 of 1910; the same concept was used more recently in patent application EP 0706826 in the name of Sangati. These solutions use a sifter which generally vibrates constantly to facilitate the passage of the product through it. However, in practice it has been found that this solution has problems, since the sifter tends to clog, after which sifting efficiency drops to entirely insufficient values. Providing maintenance for manual cleaning of the sifters in not compatible with the management criteria of the apparatus, since it would be necessary to stop the machine and this, in practice, is industrially unacceptable. Therefore, according to these solutions, after prolonged operation it is not possible to satisfactorily reduce the flow of product that passes through the second milling passage.
French Patent 1 296 235 discloses an arrangement in which a single large roller is coupled with three small rollers. After each milling step there is arranged a rotating sifting step, operating by aspiration under a vacuum. This arrangement has some problems. First of all, the fact that the sifting steps operate by aspiration causes that the screening capacity of the sifting step is very low. In fact, the aspiration of the through fraction cannot be too strong, otherwise the screened fraction cannot be released, so clogging the sifting step. A weak aspiration causes an insufficient separation because the product tends to pass without being aspired. Also this arrangement involves additional working expenses for the cost of aspiration and plant expenses for the necessary connections of the device to the aspiration and separation means. Also this arrangement teaches the use of a single milling line and not of two parallel milling lines as taught for example by EP 0706826. So a single milling line involves a capacity reduced by half. This alone is a serious drawback that nullifies the scope of stacking two milling passages in series into a single device. On the other end, it is impossible to arrange two parallel lines, according to this French patent into a single machine, because the arrangement is too cumbersome. Finally, even if more than 25 years passed from the publication of this French patent, this machine did not have any appreciable industrial exploitation so showing the lack of effective usefulness.
German patent 3327 of 1877 teaches and use of two milling steps in which the separation is arranged only after both milling steps. Between the two steps there is only arranged a brush for cleaning the rollers. So the mill according to this document is afflicted by all the above considered drawbacks for the case in which two milling steps are provided without intermediate separation.
German patent 207543 of 1906 teaches a device for milling malt for the brewing industry. This is a completely different field that operates in different ways. Particularly this document discloses a wet milling, in which water is sprayed onto the rollers during milling. In the field according to the invention it is completely unacceptable to add water to the rollers of the roller mill. So the milling conditions and separation requirements are so different that no comparison is possible. The device includes a rotating member which throws the milled product against a sieving surface. The sieving surface is arranged far from the rotating member. There is nothing that prevents a rapid clogging of the sieving surface and so a prompt decrease of the separation effectiveness.
So, none of the prior art documents has been able to solve the above problems. Furthermore, the unsolved separation problems are passed on to the downstream operations, where it is necessary to separate the product which has been milled, in practice, twice in a row. In this regard it should be noted that in all the above mentioned cases the second milling passage must be substantially different from the first one to essentially avoid useless work; that is to say, the distance of the rollers must be smaller or the number of grooves on the rollers must be increased; all these refinements entail producing an increasingly finer milled product which accordingly, for the above cited reasons, is more difficult to separate.